The present invention relates to the improvement of an electro-magnetic wave absorber device, or the improvement of a device for preventing the leakage of waves. The present device is used, for instance, for preventing leakage of wave energy in a micro-wave heater, like a micro-wave oven.
Conventionally, an absorber device for microwave leakage in a microwave oven has three absorber means. The first one is a metal contact spring which provides the conductive contact between the body and the door to close the door completely. The second one is a choke cavity with 1/4 wavelength for absorbing waves which leak through said conductive contact. The third one is a ferrite absorber provided at the outlet of the leakage path for absorbing the rest of the leakage.
The present invention relates, in particular, to the improvement of said choke cavity, and/or the combination of the choke cavity and the ferrite absorber.
FIG. 1 shows the structure of a prior wave absorber device which has said three absorber means. In the figure, the reference numeral 1 is a wall of the main body of a microwave heater, 2 is a door for closing the opening of the main body, 3 is a cabin of the main body. The elongated thin leakage path L is left between the wall 1 of the main body and the door 2. Along the leakage path L, the conductive spring 13 which provides the complete electrical contact between the wall 1 and the door 2, the choke cavity 6 provided in the door 2, and the ferrite absorber means 7 are provided. The microwave energy which tends to leak is first prevented by the spring 13, then, some portion which leaks past the spring 13 is absorbed by the choke cavity 6, and then, the rest of the microwave energy which still leaks past the choke cavity 6 is absorbed by the ferrite absorber 7 as shown by the dotted line in the figure. The choke cavity 6 has conductive walls 4 and 5 which provide an elongated closed body with the length of 1/4 wavelength. The choke cavity 6 has a window for entering waves, and said window is covered by the choke cover 8 which is made of dielectric material like polypropylene which has a small dielectric constant in order to prevent dust entering into the choke cavity 6. The reference numeral 9 is a decorative cover made of plastic, 10 is a glass window provided on the door 2, 11 is a conductive net which provides the shield effect to the glass window 10, 12 is a fixing screw, and 13 is a conductive spring for providing the conductive contact between the door 2 and the main body wall 1.
As mentioned above, waves which tend to leak are first prevented by the conductive spring 13, then, waves which leak past the spring 13 are absorbed by the choke cavity 6. The choke cavity 6 receives waves through the entrance window 6a which is covered by the dielectric body 8 which does not prevent the entrance of waves into the choke cavity 6. The waves which still leak past the choke cavity 6 are finally absorbed by the ferrite absorber 7 which is positioned next to the choke cavity 6.
However, a prior absorber device as in FIG. 1 has disadvantages as follows. First, the bandwidth of the choke cavity 6 for providing enough attenuation is rather norrow, and therefore, the size of the choke cavity for absorbing microwaves of 2450 MHz must be very accurate. If the center frequency of the choke cavity 6 for providing the maximum attenuation shifts a little from 2450 MHz, the attenuation provided by the choke cavity 6 is deteriorated considerably. Secondly, a prior choke cavity can not provide enough attenuation because of a lot of operational modes of waves in a microwave oven. Although a prior choke cavity may provide enough attenuation in an experimental device which provides a single and pure operational mode of microwave power, it cannot provide enough attenuation in an actual microwave oven which has many operational modes. Further, since the material of the ferrite absorber 7 is different from the material of the choke cover 8, the structure of the combination of the choke cavity and the ferrite absorber is complicated. In order to solve some of said disadvantages, an improved choke cavity which has wave absorber material within the cavity itself has been proposed. Although that choke cavity has a large enough bandwidth, it has the disadvantages that the attenuation of the choke cavity is deteriorated considerably due to the decrease of the value Q of the choke cavity because of the presence of the absorber material within the cavity itself, and that absorber material in the cavity might be burnt or broken because of the strong magnetic and/or electric field in the cavity.